This invention relates to sewing machines and, more particularly, to sewing machines employing stepping motors to control the placement of successive stitches to form a selected pattern.
In recent years, so called "electronic" sewing machines have gained in popularity and have met with commercial success in both industrial and domestic applications. These electronic sewing machines typically include a memory unit for storing in digital form information to control the placement of successive stitches to automatically produce a desired pattern. The stitch position coordinates may be controlled either by influencing the needle jogging and/or the work feeding or by influencing the travel of a work holder relative to the path of needle reciprocation. Sgnals generated from the stored information are applied to signal responsive actuators for selectively positioning the needle and the work feeding mechanism. These actuators may be of either the analog type or the digital type. An analog actuator is responsive to an analog signal for positioning its associated mechanism at a point along a continuum between two extreme positions. The present invention is concerned with digital actuators wherein the actuator responds to digital input signals to position its associated mechanism at a selected one of a plurality of incrementally displaced discrete points between two extreme positions. In particular, the present invention is directed to such sewing machines wherein the digital actuator includes a stepping motor, the angular orientation of the stepping motor shaft controlling the associated mechanism.
Typically, stepping motors are run in an open loop mode, that is, the different coil pairs of the stepping motor are sequentially energized a predetermined number of times to incrementally change the angular position of the rotor and hence the angular position of the output shaft connected to the rotor. However, the precise angular orientation of the shaft cannot be uniquely determined by knowing which particular set of coils is energized, because for each coil pair energization, the rotor may be aligned with the coil pair in either a first orientation or a second orientation angularly displaced 180.degree. from the first angular orientation. In stepping motors having multiple sets of coil pairs within a phase, the ambiguity of the angular orientation of the rotor is even greater.
It is therefore an object of this invention to provide an arrangement for determining the precise angular orientation of a stepping motor output shaft.
Shaft encoders per se are well known in the prior art for determining the angular orientation of a shaft. These shaft encoders typically include a plurality of sensing elements responsive to cooperating indicia on the shaft and decoding circuitry responsive to outputs from the sensing elements. These shaft encoders perform their desired function satisfactorily. However, economies can be achieved by reducing the number of sensing elements utilized. Since with a stepping motor some information is already available, i.e., which coils are energized, it would be desirable to utilize the available information to provide a more economical shaft position determining arrangement.
It is therefore another object of this invention to provide a stepping motor shaft position determining arrangement employing a reduced number of sensing elements.